Electromagnetic acoustic device



July 4, 1933. J, WHIITE ET AL 1,916,703

ELECTROMAGNETIC ACOUSTIC DEVICE Filed July 9, 1930 1-v- TOES Jack White, Edwin (1100 lm Payne,

- I BE ATTOE' i Patented July 4; 1933 UNITED PORATION or NEW JERSEY PATE T oFF1cE f-f .JYA K wHIT on PALMEBS GE En; 'LoNnoN, AND nwInMALcoLM r Y .01 SOUTHALL, ENGLAND, AssIGNoRs TO vro'ron TALKING MACHINE COMPANY, a com ELECTROMAGNETIC ncoos'rrc DEVICE Application filed July 9, 1930,Seria1 No. 466,670, and in Great Britain August 20,1929, I

'The present invention relates to electromagnetic acoustic devices such as are used for recording sound, for reproducing from sound records or for inter converting sound 5 and electrical oscillations.

It is an object of the present invention to increase the efliciency with which such devices convert electrical oscillatory energy into mechanical energy or the reverse and to make such devices more compact and of lighter weight. 7

According to the present invention an electro-magnetic acoustic device is provided with *r one or more coils in which the electrical conductor is of magnetic material. V

The invention is illustrated by way of example in the accompanying drawing which shows diagrammatic views of two forms of x the invention as applied tostylus operated V 29 devices for reproducing recorded sound, commonly known as pick-up devices.

Figs. 1 and 2 are Views in part sectional front elevation and sectional side elevation "frespectively of one form of the invention, the section of Fig. 2 being taken along the line AA of Fig. 1, and Fig. 3 is a view in part sectional front elevation of a modification of the arrangement in Figs. 1 and 2. Referring to Figs, 1 and 2, a U-shaped per- 39 manent magnet 1 has attached to one of its poles a pole piece 2 (which is preferably laminated) of U-shape, the tips of the pole piece being turned inwards leaving a small "jair gap between them. The pole piece limbs are provided above and beneath at a short distance from their tips with slots so that the tips of the pole piece are connected to the remainder by a narrow strip 3 only. Coils rrt of insulated iron wire (or of any other wire having good magnetic properties) are wound in these slots so as to fill them.- If desired the windings may project slightly beyond the upper and lower faces of the pole iipiece, in which case bobbins '5 are preferably prvoided as shown to retain the windings. The upper surfaces of the pole tips are tapered from the windings to the tips so that the area of the tips is only about one '5 quarter of the cross-sectional area of the main portion of the pole piece.

An armature 6 is mounted for vibration between the two pole tips about an axis 7, the axisbeing disposed beneath but. parallelv to the'planeof the pole piece and lying in-the plane of the permanent magnet. The bearings for the armature are not illustrated. The pole 8 of the permanent magnet is tapered and its tip is disposed close to the tip of the armature 6. Means for clamping a reproducing stylus 9 are provided upon the armature beneath the axis. force between the armature tip and the adj acent magnet pole tip 8 provides some or all of the necessary restoring force to the armature.v If desired, however, additional damping and centering means may be provided in known manner.

In the modification of the arrangement described shown in Fig. 3, the tapered tips upon the pole piece are omitted and the armature 6 co-operates with the inner faces of the coils 4 themselves. The windings in this case may be held in position by shellac or the like or bobbins 5 may be used in which case the inner cheeks of the bobbins lie between the winding (constituting the greater part of the pole piece tip) and the armature. These inner cheeks, and, if desired, the whole bobbins 'is not intended to limit the scope of the in- The magnetic vention to this arrangement. If desired, the

coils may be used instead of existing coils without removing any of the normal core. The windings may alsobe used to carry direct current for magnetizing purposesin acoustic devices.

We claim:

1. In an electro-magnetic-device having a Vibratory armature, the combination of means providing a magnetic circuit having poles associated withsaid armature, said means including a core of magnetic material having slots adjacent to said poles whereby the cross-sectional area of the magnetic circuit is reduced through said slots, and a coil of insulated wire of magnetic material in each of said slots whereby said coils are thereby included in and forming part of the magnetic circuit and serving to restore to substantially normal cross sectional area,

the magnetic circuit Within said region.

3.:11'1, an electronnagnetic device ofthe character described, the combination I of means providing a magnetic circuit having substantially a uniform cross sectional area for a portion of its length, said cross sectional area of said portion of the magnetic circuit being Wholly constituted by 'a core of reduced cross section and a Winding,hotl1 of magnetic material, the one complementary to the other in providing said uniform cross sectional area through said portion of the magnetic circuit, and an adjacent portion of the circuit of substantially the same cross sectional area ontig ous h eto eing W o y c nstitu ed by the core whereby the Winding serves to replace a portion of the magnetic core necessary to provide the cross sectionalarea of the magnetic circuit, with corresponding reduction in Weight of saidi'device. I

In testimony whereof We have signed our names to this'specification. i

JACK WHITE.

EDWIN M L 'QLM PAYNE.

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